Active retrodirective antenna array employing spiral elements and tunnel diode amplifiers



April 21, 1970 A. F. SNYDER 0 6 ACTIVE RETRQDIRECTIVE ANTENNA ARRAY EMPLOYING SPIRAL ELEMENTS AND TUNNEL ODE AMPLIFIERS Filed May 1968 /5 f5 [9 w/J Q 1 I 1 /I7 2/ Fl H fiz INVENTOR. y a a n e l/rr/rm rfi av/one 0 1' 6 BY I w 4 M;

United States Patent O 3,508,269 ACTIVE RETRODIRECTIVE ANTENNA ARRAY EMPLOYIN G SPIRAL ELEMENTS AND TUN- NEL DIODE AMPLIFIERS Anthony F. Snyder, Boonville, N.Y., assignor to the United States of America as represented by the Secretary of the Air Force Filed May 2, 1968, Ser. No. 726,190 Int. Cl. H01q 1/36 US. Cl. 343701 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Retrodirective antenna arrays are commonly used to improve communication signals passing between transmitting and receiving points which are otherwise blocked fromeach other. Also, with appropriate signal modulation means, retrodirective antenna arrays find utility in radar beacon systems. Such applications often call for installation of the antenna array in aircraft. Simplification of circuits, reduction of weight, improved reliability and maximum range capability are, therefore, important design objectives. The retrodirective antenna array, with proper amplification, meets these design objectives. How ever, approaches used in prior art systems to achieve adequate amplification have been something less than satisfactory. The use of unilateral amplifiers requires that one set of antennas be used for receiving and a second set be used for transmitting. This method subtracts from the available array aperture due to the additional antenna elements required. Amplification has also been achieved with only two antenna elements per retrodirective pair by separating the incident energy and the energy to be reradiated through the use of separate transmission lines, ferrite isolation, frequency blocking networks, mixers and unidirectional amplifiers. The added number of components and greatly increased weight and expense of such amplification means make it undesirable for airborne amplification. The present invention is directed toward overcoming these and other deficiencies prevalent in prior art systems.

SUMMARY OF THE INVENTION The present invention comprehends the integration of bilateral tunnel diode amplifiers into the interconnecting transmission lines of a retrodirective antenna array. The purpose of a retrodirective antenna array is to return the incident electromagnetic energy in the direction of the transmitting source. The subelements in the array consist of 2 equiangular spiral antennas connected by a balanced strip transmission line. At the midpoint between antenna elements a bilatral tunnel diode amplifier is integrated into the strip transmission line.

In operation, the electromagnetic energy, of frequency within the design constraints of the array, incident upon the dual antenna aperture gives rise to current and voltage flow in the interconnecting transmission line. The tunnel diode, when placed across the line, represents a discontinuity in the line that results in a reflected and a forward travelling wave at the diode boundary. The discontinuity, having a negative resistance value when properly biased, imparts energy to both the reflected and forward travelling wave. There are two antennas connected to each diode. The resulting terminal current at each antenna is due to a component that originated at its self and was amplified by the diode during reflection, plus a component that was launched down the line from the other member of the antenna pair and was amplified by the diode during its forward flow across the diode boundary.

The resulting terminal current for each antenna has proper phase to construct two equiphase fronts (principal maxima) travelling away from the total array aperture. One maxima represents a mirror reflection term while the other gives the desired retrodirective characteristics. The two maxima appear as a result of the unique characteristics of the tunnel diode amplifier used. The wave that experiences transmission amplification gives rise to the retrodirective scattering while the wave that is reflected and amplified forms the mirror scattering (characteristic of arrays of independent antennas terminated in an effective negative resistance).

It is a principal object of the invention to provide an improved active, retrodirective antenna array that is capable of reradiating energy of an amplitude greater than that received.

It is another object of the invention to provide an active retrodirective antenna array that is lightweight and inexpensive.

It is another object of the invention to provide an active retrodirective antenna array having a minimum number of components and improved efliciency of operation.

It is another object of the invention to provide. an active retrodirective antenna array having signal amplification provided by a single tunned diode per antenna element pair.

These, together with other objects, advantages and features of the invention will become more apparent from the following detailed description when taken in conjunction with the illustrative embodiments in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a view, in elevation, of an active retrodirective antenna array incorporating the principles of the present invention;

FIGURE 2 is a view of the antenna array of FIGURE 1 taken at 2-2;

FIGURE 3 is a view of the antenna array of FIGURE 1 taken at 3-3;

FIGURE 4 is an enlarged sectional view of the antenna array of FIGURE 1 taken at 44 and illustrates in detail the bilateral amplifier and associated susceptance cancelling means of the invention;

FIGURE 5 is a schematic representation of one means for biasing the tunnel diode amplifiers of the invention; and

FIGURE 6 is an equivalent circuit of a typical bilateral tunnel diode amplifier of the type comprehended by the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A presently preferred embodiment of the invention is illustrated in the accompanying drawing by FIGURES l, 2 and 3. The retrodirective antenna array 7 therein disclosed comprises antenna elements 9, 10, 11 and 12,

)alanced strip transmission lines 17 and 18 and tunnel liodes 19 and 22. Antenna elements 9 and 12, together with interconnecting transmission line 17, comprises a tubelement as do antenna elements 10 and 11 and transnission line 18. Although only two pairs of antenna elenents are described, it is to be understood that arrays 1aving any number of such subelements are com- ;rehended by the invention.

Antenna arrays of the type herein described can be :onveniently fabricated by etching antenna elements and ransmission lines from copper clad insulating sheet maverial. Thus, in FIGURES 1, 2 and 3, transmission lines [7 and 18 are etched copper strips residing on both sides at sheet insulating material 8 and constitute balanced ;trip transmission lines. Insulating sheet material 8 may be my suitable low dielectric factor plastic such as that :ommercially available under the trade name Rexolite [422. Antenna elements 9, 10, 11 and 12 are likewise :tched from copper plated insulating sheets 13, 14, 15 and 16. Electrical connections between transmission lines and antenna elements are made in a conventional manner.

It is requisite that conjugate antenna elements be posizioned equidistant from the array center and that the transnission lines be some integral number of wavelengths ong. It is also necessary that the tunnel diodes 19 and 22 be integrated into transmission lines 18 and 17 at he midpoint between antenna elements. These tunnel liodes in combination with open circuit stubs 25 and 26 tnd their associated resistors 20, 21 and 23, 24 provide amplification for the subelements. FIGURE discloses a :onvenient means for biasing tunnel diodes. Such a means :omprises a DC. bias source 31, current limiting resistors 29 and 30 and switch 32. The bias is applied as illustrated the outer edges of equiangular spiral antenna elements 57 and 28.

The tunnel diodes and their associated resistors can :e integrated into the balanced strip transmission lines in he manner illustrated in FIGURE 4. Resistors 21 and 52 are inserted into insulating sheet 8 a distance of a quarter wavelength from the open end of stub 25 and a listance I from tunnel diode 19. Electrical connections are made to strip transmission line 18 as shown.

The equiangular spiral antenna is an optimum antenna ype for tunnel diode stabilization. The unconditional tability criteria, for preventing diode oscillations,

RD RT vhere R =the negative resistance of the diode when biased in the minus resistance region =total circuit resistance,

lemands that the proper resistive load be presented to the unnel diode below its resistive cutoff. The low frequency tabilizing resistors (such as resistor 20, 21 and 23, 24 If FIGURE 1) are an effective load up to design freuency of the amplifier. The antenna impedances must act [8 the stabilizing elements from the design frequencies of he amplifier to the resistor cutoff of the tunnel diode. Fhe equiangular spiral antenna is considered a frequency ndependent structure because its effective arm lengths II'C entirely specified by angles. A frequency change, herefore, is an effective rotation in the plane of the tntenna. Thus, the impedance of the antenna is considered :onstant for bandwidths of greater than 20:1. The upper requency limits of such antennas are determined by the eed point construction.

In accordance with the principles of the invention, the mpedance of each transmission line is matched to its tntenna. Also, the line lengths between each diode and its tssociated antenna elements are made an integral num- )er of wavelengths long. This eliminates any large antenna mpedance transformation over the line length to the diode. Fora balanced strip transmission line the characteristic impedance- Z can be calcuated using the formulas:

where W is the width of the strip transmission line, and S is the thickness of the insulating material separating them.

Amplification in each subelement is achieved by placing a tunnel diode between the balanced transmission line at the midpoint between the two antennas (such as tunnel diodes 19 and 22 of FIGURE 1). The equivalent circuit for such a tunnel diode is shown in FIGURE 6. Having reference now to FIGURE 6, C represents the diode package capacity, L the diode series inductance, R the diode series resistance, C,- the diodejunction capacity, and R the diode negative resistance. The diode admittance YD is The imaginary term in this equation is the effective susceptance jY of the diode.

It is necessary to tune this susceptance out for proper amplifier operation. To do this, an open circuit line is connected at right angles to the balanced lines containing the diode. This is illustrated in FIGURE 1 by stubs 25 and 26. One quarter of a wavelength from the open circuit, resistors 20, 21 and 23, 24 are put in parallel across the lines to stabilize tunnel diodes 19 and 22 at lower frequencies. Because the resistors are located one-quarter of a wavelength from the open circuit, they are effectively short circuited at the operating frequency of the amplifier. The necessary line length required to cancel the susceptance of each diode is determined and this is the line length between the diode and its associated stabilizing resistors. The admittance of a short circuited transmission line Y can be written Therefore, the required tuning length I can be calculated from jY jOOC =jY Cot where C is the effective diode capacitance Y =the stripline characteristic admittance W/21r=the frequency of operation B==21r/)\ This is the only tuning required as the antenna impedances are real and have no reactive component.

Referring now to FIGURE 5, there is illustrated thereby means for providing appropriate bias for the tunnel diode amplifiers. The diode biasing circuit must be isolated from the amplifier circuitry to eliminate any amplifier detuning and R-F conduction in the bias leads. Such isolation is accomplished by feeding the bias voltage through the end points of the spiral antennas. Isolation is produced because of the rapid attenuation of the RF fields near the end points of this antenna type. The expression for the individual antenna terminal current is where:

Y =diode admittance Y =Antenna admittance I =Effective Norton current source I =Total terminal current for the antenna.

' This shows that as Y /2 approaches the value of Y,,, large 5 thereby enhancing the energy scattered from the antenna.

Although the invention disclosed herein has been described with reference to specific embodiments, it is not intended that it be so limited. Various modifications such as the use of coaxial cable or unbalanced strip transmission lines and variations to the amplifier tuning technique can be made without departing from the true scope of the invention. Accordingly, it is to be understood that the scope of the invention shall be limited by the claims only.

What is claimed is:

1. An active retrodirective antenna array comprising at least one pair of antenna elements, transmission line means engaged between conjugate elements of each antenna element pair, a bilateral amplifier integrated into each said transmission line means at the midpoint thereof, and susceptance cancelling means integrated into each said transmission line means, said susceptance cancelling means being adapted to cancel the effective susceptance of each said amplifier.

2. An active retrodirective antenna array as defined in claim 1 wherein each said antenna element comprises an equiangular spiral antenna.

3. An active retrodirective antenna array as defined in claim 2 wherein each said transmission line means comprises a microwave strip transmission line.

4. An active retrodirective antenna array as defined in claim 3 wherein said bilateral amplifier comprises a tunnel diode.

5. An active retrodirective antenna array as defined in claim 4 wherein each said susceptance cancelling means comprises an open circuited stub located at midpoint on said transmission line means and at right angle thereto and resistance means incorporated within said open circuited stub, said resistance means being adapted to stabilize said tunnel diode at low frequencies.

References Cited UNITED STATES PATENTS 3,296,536 1/1967 Copeland et al. 343-701 ELI LIEBERMAN, Primary Examiner U..S. Cl. X.R. 

